If you do this correctly, the ammunition counter in the bottom corner of the screen should switch to the tank shells counter and image. Furthermore, you'll be able to fire the big gun—the shell appears roughly where James Bond's mouth should be and behaves like an extremely large grenade launcher, just as it should.

If you switch away from the tank gun, you'll be able to recover it by hopping back into the tank and repeating the process.

amib says (The node title) is also an EGM quote about the all-weapons code in 007 Nightfire. If you activate it, you can fire the tank gun without the tank. And now you know.